Normally, the surface to be acted upon is covered by a protective stencil made of flexible rubber or plastic which is adhesively attached to the surface to be forcefully sand blasted by the abrasive material. The stencil has a cut-out configuration desired to be acted upon by the forceful blast of abrasive material.
The finely-divided abrasive material blast is produced by a sandblasting jet or jets of pressurized air carrying the abrasive material, directed against the exposed cut-out portions of the stencil-covered surface. The cut-out portions of the stencil are acted upon by the blast to engrave the surfaces therebeneath. The protected surfaces underneath the solid or non-cut-out portions of the stencil are not acted upon by the blast.
Prior art sand blasting processes have previously been performed in a suitable enclosure to receive the workpieces to be acted upon. Such enclosures have commonly had an access opening on one side. Usually a blast-resistant curtain having a horizontal slot is hung over the access opening and is movable vertically with respect to the enclosure. A single sandblast jet is inserted through this slot and moved back and forth horizontally as the curtain is moved vertically. In the majority of such operations the jet is manually operated and the workpiece is mounted vertically.
It has been found that manual operation of the jet by most skillful operators has been unable to produce sandblasting effects of uniform quality or depth of penetration of precise magnitude. Since the jet cannot be manually operated with precisely uniform motion, the workpieces are frequently cut too deeply in some areas and not sufficiently deep in other areas.
In addition, such manual operations are harmful to the operators as the sand, or other harmful substances in the abrasive material, can be inhaled by the operators who in time can and do contract lung diseases.
As stated, various types of automatic sand blasting machines have been developed which include a horizontally-movable carriage and a vertically-movable curtain having a slot-type opening therein, the sand blast jet being located on or through the carriage and projecting through the slot in the curtain. Such machines have inherent difficiencies in the combinations of carriages and curtains to permit the blasting jet to operate therethrough. Also such machines only employ a single jet and are often modified to permit manual or semi-manual operation resulting in inefficient operation. Further, the resulting end products are frequently of poor quality engraving primarily depending on the skill of the operator.
In the case of stone memorial markers, over substantial periods of time, the markers do not wear evenly and the engraved indicia tend to become illegible in the shallower engraved areas. Thus, precisely uniform engraving is highly desirable in high-quality workpieces which can withstand the tests of time in outdoor exposure.
U.S. Pat. No. 3,436,866 to Nye discloses an automatic sandblasting machine in which an upright rectangular frame is mounted parallel to and spaced from the front of the curtain. A horizontally-mounted elongated carriage is vertically movably mounted in the frame and means is provided attaching the carriage to the curtain. The carriage is moved up and down in the frame and the curtain is moved therewith. A second movable carriage is also provided.
In this type of sandblast machine, a single blast jet is employed to move with the curtain and within the curtain slot to direct the single jet against the work surface. The curtain-type machine is impractical in modern-day use wherein the waste products of sand blasting, usually air-borne dust, must be fully contained and are not permitted to pass into the atmosphere causing health and contamination problems. In addition, the single jet cannot engrave any substantial surface of a workpiece to a uniform depth of high quality.
U.S. Pat. No. 2,617,225 to O'Brien discloses a method of sandblasting employing a single venturi-type nozzle and not a direct-air-type nozzle of the present invention. The nozzle of this prior art method is manually controlled and cannot provide high-quality engraving of uniform depth.
U. S. Pat. No. 2,450,401 to Thompson also utilizes a single venturi-type nozzle and has the same deficiencies of the other prior art.